High fidelity doping paste and methods thereof

ABSTRACT

A high-fidelity dopant paste is disclosed. The high-fidelity dopant paste includes a solvent, a set of non-glass matrix particles dispersed into the solvent, and a dopant.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. application Ser. No.12/967,654, filed December 14, 2010.

FIELD OF DISCLOSURE

This disclosure relates in general to semiconductors and in particularto a high fidelity doping paste and methods thereof.

BACKGROUND

Semiconductors form the basis of modern electronics. Possessing physicalproperties that can be selectively modified and controlled betweenconduction and insulation, semiconductors are essential in most modernelectrical devices (e.g. computers, cellular phones, photovoltaic cells,etc.).

Typical solar cells are formed on a silicon substrate doped with a firstdopant (the absorber region), upon which a second counter dopant isdiffused using a gas or liquid process (the emitter region) completingthe p-n junction. After the addition of passivation and antireflectioncoatings, metal contacts (fingers and busbar on the emitter and pads onthe back of the absorber) may be added in order to extract generatedcharge carriers. Emitter dopant concentration, in particular, must beoptimized for both carrier collection and for contact with the metalelectrodes.

Electrons on the p-type side of the junction within the electric field(or built-in potential) tend to be attracted to the n-type region(usually doped with phosphorous) and repelled from the p-type region(usually doped with boron), whereas holes within the electric field onthe n-type side of the junction may then be attracted to the p-typeregion and repelled from the n-type region. Generally, the n-type regionand/or the p-type region can each respectively be comprised of varyinglevels of relative dopant concentration (e.g., phosphorous, arsenic,antimony, boron, aluminum, gallium, etc.) often shown as n−, n+, n++,p−, p+, p++, etc. The built-in potential and thus magnitude of electricfield generally depend on the level of doping between the adjacentlayers.

In some solar cell architectures, it may be beneficial to alter the typeand concentration of a dopant as a function of substrate position. Forexample, for a selective emitter solar cell, a low concentration of(substitutional) dopant atoms within an emitter region will result inboth low recombination (thus higher solar cell efficiencies), and poorelectrical contact to metal electrodes. Conversely, a high concentrationof (substitutional) dopant atoms will result in both high recombination(thus reducing solar cell efficiency), and low resistance ohmic contactsto metal electrodes. One solution, typically called a dual-doped orselective emitter, is generally to configure the solar cell substratewith a relatively high dopant concentration in the emitter regionbeneath the set of front metal contacts, and a relatively low dopantconcentration in the emitter region not beneath the set of front metalcontacts. Differential doping may also be beneficial to other solar cellarchitectures where the dopant needs to be localized, such as a backsidecontact solar cell.

Referring now to FIG. 1, a simplified diagram of a conventional solarcell is shown. In general, a moderately doped diffused emitter region108 is generally formed above a relatively light and counter-dopeddiffused region absorber region 110. In addition, prior to thedeposition of silicon nitride (SiN_(x)) layer 104 on the front of thesubstrate, the set of metal contacts, comprising front-metal contact 102and back surface field (BSF)/back metal contact 116, are formed on andfired into silicon substrate 110.

In a common configuration, a light n-type diffused region 108 (generallycalled the emitter or field), is formed by exposing the boron-dopedsubstrate to POCl₃ (phosphorus oxychloride) ambient to formphosphosilicate glass (PSG) on the surface of the wafer. The reductionof phosphorus pentoxide by silicon releases phosphorus into the bulk ofthe substrate and dopes it. The reaction is typically:

4POCl_(3(g))+3O_(2(g))→2P₂O_(5(l))6Cl_(2(g))   [Equation 1A]

2P₂O_(5(l)+)5Si_((s))→5SiO_(2(s)+)4P_((s))   [Equation 1B]

Si+O₂→SiO₂   [Equation 2]

The POCl₃ ambient typically includes nitrogen gas (N₂ gas) which isflowed through a bubbler filled with liquid POCl₃, and a reactive oxygengas (reactive O₂ gas) configured to react with the vaporized POCl₃ toform the deposition (processing) gas. In general, the reduction of P₂O₅to free phosphorous is directly proportional to the availability of Siatoms.

Referring now to FIG. 2, a simplified diagram of a selective emitter isshown. In general, a relatively heavy n-type diffused region (highdopant concentration) 214 is generally formed in emitter areas beneaththe set of front-metal contacts 202, while a relatively light n-typediffused region (low dopant concentration) 208 is generally formed inemitter areas not beneath the set of front-metal contacts 202. Inaddition, prior to the deposition of silicon nitride (SiN_(x)) layer 204on the front of the substrate, the set of metal contacts, comprisingfront-metal contact 202 and back surface field (BSF)/back metal contact216, are formed on and fired into silicon substrate 210. In a commonconfiguration, light n-type diffused region 208 (generally called theemitter or field), is formed by exposing the boron-doped substrate toPOCl₃ as previously described.

In an alternate configuration to those in FIGS. 1 & 2, the diffusion maybe formed (or partially formed) using a doping paste directly depositedon the surface of the substrate, instead of through an ambient gassource. In general, an n-type or p-type dopant source is combined withsome type of matrix material, preferably printable, that both providesthe dopant source in a deposited pattern during the diffusion process,and is subsequently easily removed once the diffusion process hascompleted.

N-type doping pastes may include dopant precursors such as n-typeliquids (i.e., phosphoric acid [H₃PO₄], organophosphates[O═P(OR)_(x)(OH)_(3-x)], etc.), n-type solids (i.e., P₂O₅, inorganicphosphates [Na₃PO₄, AlPO₄, etc.] and phosphides [AlP, Na₃P, etc.]), andn-type polymers (i.e., polyphosphonates, polyphosphazenes, etc.).

P-type doping pastes may include dopant precursors such as p-typeliquids (i.e., borate esters [B(OR)₃]), p-type solids (i.e., boric acid[B(OH)₃], borates [NaBO₂, Na₂B₄O₇, B₂O₃]), p-type binary compounds(i.e., boronitride, boron carbide, boron silicides and elementaryboron), and p-type polymers (i.e., polyborazoles, organoboron-siliconpolymers, etc.).

An example of a common matrix material is a silica sol-gel. A “sol” istypically a stable suspension of colloidal particles within a liquid(2-200 nm), and a “gel” is a porous 3-dimensional interconnected solidnetwork that expands in a stable fashion throughout a liquid medium andis limited by the size of the container.

In general, the sol-gel derived glass formation process involves firstthe hydrolysis of the alkoxide (sol formation), and second thepolycondensation of hydroxyl groups (gelation). For a given siliconalkoxide of general formula Si(OR)₄, R being an alkyl chain, thesereactions can be written as follows:

Hydrolysis

Si(OR)₄+H_(2O)→(HO)Si(OR)₃+R—OH   [Equation 3]

Condensation

(H_(O)Si)(OR)3+Si(OR)4→(RO)₃Si—O—Si(OR)3+R—OH   [Equation 4]

(OR)₃Si(OH)+(HO)Si(OR)₃→(RO)₃Si—O—Si(OR)₃+H₂O   [Equation 5]

For example, a sol-gel suspension (comprising silicon alkoxide) may becombined with an n-type precursor of phosphorus pentoxide (P₂O₅), likephosphoric acid (H₃PO₄), an organophosphate (O═P(OR)_(x)(OH)₃,) etc.Likewise, p-type doping, the sol-gel suspension may be combined with ap-type precursor of boron trioxide (B₂O₃), like boric acid (B(OH)₃),boron alkoxides (B(OR)₃), etc.. The resulting doped silicon glass(phosphoro-silicate glass (PSG) and boro-silicate glass (BSG) for n-typeand p-type doping respectively) formed by condensation reaction duringhigh temperature bake (200° C.<T_(bake)<500° C.) is used for subsequentdopant diffusion process.

However, the use of a sol-gel doping paste may be problematic forselective doping due to relatively low glass transition temperature ofdoped silicon glasses. Additionally, the glass transition temperaturetends to decrease significantly with an increasing dopantcontencentration corresponding to increasing atomic disorder of thesilica layer. See J. W. Morris, Jr., Chapter 5: Glasses, Engineering 45Notes, Fall 1995, UC Berkeley.

The glass transition temperature of doped silica glass formed from atypical doping paste is substantially below temperature needed to drivethe dopant into the silicon substrate. As a result, the doped silicaglass tends to reflow during high temperature processing resulting inspreading of the dopant source on the surface. While not problematic(and perhaps even beneficial) for the blanket doping of large substratessurfaces, the use of a doping process that produces a silicon glass isproblematic for the forming of high-fidelity doping regions, such aswould be required under the front metal fingers to form an ohmiccontact.

In addition, many typical sol-gel doping pastes have sub-optimal screenprinting characteristics. In general, in order to be commercially viablein high-volume solar cell production with a high printing resolution, apaste used in a screen printer must be a non-Newtonian shear-thinningfluid. Non-Newtonian fluid refers to a fluid whose flow properties arenot described by a single constant value of viscosity. Shear thinningrefers to a fluid whose viscosity decreases with increasing rate ofshear stress.

Consequently, the viscosity of the paste must be relatively low at highshear rates in order to pass through a screen pattern, but must berelatively high prior to and after deposition (at low or zero shearrates), in order not to run through the screen or on the substratesurface respectively. However, many typical sol-gel doping pastesexhibit a near-Newtonian behavior, which means that they are either tooviscous to effectively pass through a screen, or not viscous enough toprevent running, which corresponds to a low fidelity deposited pattern.

In view of the foregoing, there is desired a doping paste with a glasstransition temperature substantially greater than the relevant dopingtemperature.

SUMMARY

The invention relates, in one embodiment, to a high-fidelity dopantpaste. The high-fidelity dopant paste includes a solvent, a set ofnon-glass matrix particles dispersed into the solvent, and a dopant.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 shows a simplified diagram of a conventional solar cell;

FIG. 2 shows a simplified diagram of a selective emitter solar cell;

FIGS. 3A-D show a set of simplified diagrams comparing the reflow on asilicon substrate of a set of doped glasses to a silicon ink;

FIGS. 4A-B compare the viscosity vs. shear rate, and the resulting linewidth after deposition with the same screen, for both a n-typeconventional doping paste and a n-type high fidelity doping paste, inaccordance with the invention;

FIG. 5 compares line width between a conventional doping paste and ann-type high-fidelity doping paste, in accordance with the invention;

FIG. 6 shows a simplified diagram of a back-to-back dopant diffusionconfiguration for use with the high fidelity doping paste, in accordancewith the invention;

FIG. 7 compares the sheet resistance of a HF doping paste to aconventional n-type doping paste on a set of p-type silicon substrates,in accordance with the invention;

FIG. 8 compares the sheet resistance of various HF (phosphorous) dopingpaste configurations on a set of p-type silicon substrates, inaccordance with the invention; and,

FIGS. 9A-B compare the sheet resistance of various HF (boron) dopingpaste configurations on a set of n-type silicon substrates, inaccordance with the invention.

DETAILED DESCRIPTION

The present invention will now be described in detail with reference toa few preferred embodiments thereof as illustrated in the accompanyingdrawings. In the following description, numerous specific details areset forth in order to provide a thorough understanding of the presentinvention. It will be apparent, however, to one skilled in the art, thatthe present invention may be practiced without some or all of thesespecific details. In other instances, well known process steps and/orstructures have not been described in detail in order to notunnecessarily obscure the present invention.

In an advantageous manner, a high-fidelity doped pattern may be formedon the substrate by a high fidelity doping paste that includes a dopantsource(precursor) and a set of matrix particles with a high meltingtemperature (i.e., substantially greater than the diffusiontemperature). In one configuration, the set of matrix particlescomprises non-glass forming particles.

In another configuration, the set of matrix particles is dispersed in asolvent with a boiling point above 200° C. Examples of such solventsinclude, solvents with a linear or cyclic structures, solvents withsaturated or unsaturated hydrocarbon parts, hydrocarbon-based solvents(i.e. alkane, alkene, alkyne), alcohols, thiols, ethers, esters,aldehydes, ketones, or a solvent with combinations of thereof.

In another configuration, the average diameter of the set of matrixparticles is less than 25 microns. In another configuration, a binder isalso added to the solvent. In another configuration, the binder is oneof a polyacrylate, a polyacetal, a polyvinyl, a cellulose (including itsethers and esters), and copolymers thereof.

In general, a typical dopant drive in temperature is between about 800°C. and about 1050° C. (i.e., the temperature at which the correspondingdopant is driven into the substrate for substitutional bonding in thecrystalline silicon). Consequently, the melting temperature of thenon-glass matrix material should be greater than 1050° C. in order tominimize any change in shape or resolution of the deposited patternduring the diffusion process. Examples of non-glass matrix particlesinclude ceramics (i.e., Al₂O₃, MgO, CeO₂, TiO2, Y₂O₃, ZnO, ZrO₂,ZrO₂₋₃,Y₂O₃), W and WC, and elemental compounds like Carbon and Silicon.

The addition of a dopant precursor, as previously described, has minimaleffects on the melting temperature of non-glass matrix particles. Forexample, crystalline silicon has high melting temperature of about 1440°C., and the incorporation of boron up to the solid solubility limit canonly reduce the melting point to between 50-80° C. [R. W. Olesinski andG. J. Abbaschian, The B-Si System, Bull. Alloy. Phase Diagrams, 5 (no.5), 1984, p478-484; A, I. Zaitsev and A. A. Koentsov, ThermodynamicProperties and Phase Equilibria in the Si-B System J. Phase Equilib. 22(no. 2), 2001, p126-135]. Likewise, incorporating phosphorous atoms intothe silicon substrate matrix (again up to the solid solubility limit)tends to reduce the melting temperature to 1180° C. [R. W. Olesinski, N.Kanani, G. J. Abbaschian, The P-Si System, Bull. Alloy Phase Diagrams 6(no. 3), 1985].

In addition, unlike typical sol-gel doping pastes, a high-fidelitydoping paste that comprises micron and sub-micron particle sizes alsotends to exhibit strong shear-thinning (non-Newtonian) behavior. Apreviously described, non-Newtonian fluid refers to a fluid whose flowproperties are not described by a single constant value of viscosity.Shear thinning refers to a fluid whose viscosity decreases withincreasing rate of shear

Referring now to FIGS. 3A-D, a set of simplified diagrams is shown,comparing the reflow on a silicon substrate of a set of doped glasses(as used in a conventional doping paste) to a high-fidelity doping pastecomprising a silicon ink, in accordance with the invention. In general,a silicon ink, is a non-Newtonian silicon nanoparticle colloidaldispersion. More detailed information is described in U.S. patentapplication Ser. No. 12/493,946 entitled Sub-Critical Shear ThinningGroup IV Based Nanoparticle Fluid, filed on Jun,. 29, 2009, the entiredisclosure of which is incorporated by reference.

FIG. 3A compares the reflow angle φ to the reflow temperature ° C. for aset of doped glasses. Reflow temperature in ° C. is shown along thehorizontal axis 302, while reflow angle φ is shown along the verticalaxis. Upon deposition, a reflow angle φ is formed between the air-glassboundary and the glass-substrate boundary of the fluid. By definition,as a fluid spreads out, the corresponding reflow angle φ decreases.

Doped silicon glasses are deposited and then heated from about 810° C.to about 890° C. A first silicon glass 308 is comprised of 5%phosphorous and 3% boron. A second silicon glass 310 is comprised of 5%phosphorous and 4% boron. A third silicon glass 312 is comprised of 5%phosphorous and 5% boron. As can be seen, for any given temperature inthe flow range of 810° C. to about 890° C., a higher dopantconcentration corresponds to a smaller reflow angle φ. That is, tohigher wetting on the substrate surface and degradation of patternfidelity.

FIG. 3B compares the normalized line width for the same set of dopedglasses with doped silicon ink, in accordance with the invention. Thederivation of normalized line width from reflow angle is described inFIG. 3C below.

A first silicon glass 328 is comprised of 5% phosphorous and 3% boron. Asecond silicon glass 330 is comprised of 5% phosphorous and 4% boron. Athird silicon glass 332 is comprised of 5% phosphorous and 5% boron. Inaddition, a silicon ink 326 is comprised of a 10% phosphorous dopantconcentration.

As in FIG. 3A, for the set of glasses across any given temperature inthe flow range of 810° C. to about 890° C., a higher dopantconcentration corresponds to a higher difference in normalized linewidth. However, in an advantageous manner, the silicon ink 326 shows nosubstantive change in normalized line width across the same temperaturerange.

FIGS. 3C-D derive the conversion from reflow angle to normalized linewidth. See J. E. Tong, et al., Solid State Tech., January 1984, p161. Ingeneral, a deposited fluid droplet 344, such as a deposited paste orsilicon ink, may be modeled as a lateral slice 342 of a cylinder. Thefollowing derivation shows the conversion of reflow angle φ in FIG. 3Ato nominal line width 314 in FIG. 3B.

Modeling the shape of the deposited fluid as a slice of a cylinder,radius R may be calculated.

$\begin{matrix}\begin{matrix}{A = {\pi \; {R^{2} \cdot \left( \frac{2\alpha}{2\pi} \right)}}} \\{= {\alpha \; R^{2}}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 6A} \right\rbrack \\\begin{matrix}{A = \frac{x \cdot \left( {R - h} \right)}{2}} & {x = {{R \cdot \sin}\; \alpha}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 6B} \right\rbrack \\\begin{matrix}{A = \left( \frac{{R \cdot \sin}\; {\alpha \cdot \left( {R - h} \right)}}{2} \right)} \\{= \frac{{R^{2} \cdot \sin}\; {\alpha \cdot \cos}\; \alpha}{2}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 6C} \right\rbrack \\{A = {{R^{2}\sin} \propto \cos \propto}} & \left\lbrack {{Equation}\mspace{14mu} 6D} \right\rbrack \\{A = {\propto {R^{2} - {R^{2}\sin}} \propto \cos \propto}} & \left\lbrack {{Equation}\mspace{14mu} 6E} \right\rbrack \\{R = \left( \frac{1}{\propto {- \sin} \propto \cos \propto} \right)^{1/2}} & \left\lbrack {{Equation}\mspace{14mu} 6F} \right\rbrack \\\begin{matrix}{X = {R\; \sin \; \alpha}} \\{= {{\left( \frac{1}{\propto {- \sin} \propto \cos \propto} \right)^{1/2} \cdot \sin}\; \alpha}}\end{matrix} & \left\lbrack {{Equation}\mspace{14mu} 6G} \right\rbrack\end{matrix}$

Rheology Comparison

Experiment 1

Referring now to FIGS. 4A-B, set of simplified diagrams compare aconventional doping paste to the high-fidelity doping paste, inaccordance with the invention.

FIG. 4A shows viscosity vs. shear rate for both an n-type Ferro dopingpaste 406 (glass) and a n-type silicon ink based high-fidelity dopingpaste 408 (non-glass), in accordance with the invention. Log of Shearrate in 1/sec 402 is shown along the horizontal axis, while Log ofViscosity 404 measured in cP at 25° C., is shown along the verticalaxis. Viscosity was measured as a function of shear rate to show theinfluence of silicon nanoparticles on the flow behavior of the paste. Ascan be seen, conventional doping paste 406 shows typical near-Newtonianbehavior due to low surface interaction of the sol particles. Thus, theviscosity of the fluid change slightly under differing shear rate.However, silicon ink based high-fidelity doping paste 408, characterizedby significant particle-particle interaction, shows a much strongershear thinning behavior. Increased shear thinning behavior results inbetter ink flow through the screen with reduced spreading of printedfeatures on the target surface, as it can be seen in FIG. 4B.

FIG. 4B. shows line width 403 for conventional Ferro n-type paste andsilicon ink based high-fidelity doping paste after deposition with thesame screen. Two p-type substrates were each cleaned with a hydrofluoricacid/hydrochloric acid mixture prior to paste deposition. Both pasteswere deposited with a screen mask opening of 175 μm, and were then bakedat 200° C. for a time period of about 3 minutes in order to removesolvents and densify the deposited paste. As can be seen, the highfidelity doping paste can be deposited with a smaller absolute linewidth than the conventional doping paste due to stronger shear thinningbehavior. The median line width is about 365 μm for the conventionaldoping paste vs. a median line width of 224 μm for high fidelity dopingpaste. As compared to the finger opening of 175 μm, the high fidelitypaste spreads by ˜50 μm as compared to 190 μm for the conventionaldoping paste.

Experiment 2

FIG. 5 compares the average line width at different stages of processingbetween a conventional (Ferro n-type) doping paste (see Matthew Edwards,y, Jonathan Bocking, Jeffrey E. Cotter and Neil Bennett; Prog.Photovolt: Res. Appl. 16 (1) pp 31-45, 2008) and an n-type high-fidelitydoping paste on an ISO textured substrate, in accordance with theinvention.

The conventional dopant paste was printed through a screen opening of400 μm (504 a) resulting in a printed line width of ˜520 um (506 a),then baked at 300° C. for a time period of 1-2 min in order to removesolvents and densify the deposited paste, resulting line width of about570 μm (508 a), an increase from the screen mask opening of about 40%.The conventional dopant paste was heated to a temperature of 950° C. fora time period of 90 min in order to diffuse the dopant into thesubstrate, resulting in a dopant width of about 870 μm (510 a), or217.5% of the original screen opening.

For the high fidelity doping paste, a screen mask opening of 175 μm (504b) was used to deposit the paste, resulting in a deposited width ofabout 220 μm (506 b), an increase of about 20%. The conventional dopantpaste was then baked at 200° C. for a time period of about 3 minutes inorder to remove solvents and densify the deposited paste. However, theresulting line width remains about 20% larger than the screen opening(508 b). The high fidelity dopant paste was then heated to a temperatureof 950° C. for a time period of 90 min in order to diffuse the dopantinto the substrate. As before, and unlike the conventional doping paste,the resulting line width remains at about 120% of the screen makeopening (510 b).

Doping Comparison

FIG. 6 shows a simplified diagram of a back-to-back dopant diffusionconfiguration for use with the high-fidelity doping paste, in accordancewith the invention

Silicon substrates 604 are vertically positioned in a back-to-backconfiguration in order to minimize the effect of ambient dopant thatbecomes volatile from the doping paste during the doping environment.The p-type silicon substrates 604 were vertically placed back-to-backwithin a quartz tube in a horizontal diffusion furnace in order to coverthe deposited Ferro doping paste and high-fidelity doping paste 606 (asappropriate) with a corresponding substrate in an with an N₂ ambient.

Experiment 3

Referring now to FIG.7, a simplified diagram comparing the sheetresistance of a HF doping paste to a Ferro n-type doping paste on a setof (2 ohm-cm/180 μm/saw damage etched) p-type silicon substrates, inaccordance with the invention. The inventors believe that the dopingprofile Ferro doping paste is substantially similar to most dopingpastes.

The high-fidelity doping paste was prepared by addition of 10% ofphosphoric acid to Si nanoparticle paste containing 1.5 wt % ethylcellulose binder and 8 wt % silicon nanoparticles in a terpineolsolvent, followed by thorough mixing with a planetary mixer. Theconventional (Ferro) doping paste was used unmodified. The set of p-typesilicon substrates were each cleaned with a hydrofluoricacid/hydrochloric acid mixture prior to paste deposition.

The Ferro paste and the high-fidelity doping paste were each depositedon three separate substrate subsets. The substrate subset with thehigh-fidelity doping paste was then baked in N₂ ambient at 200° C. for 3minutes to densify the film and to dehydrate the phosphoric acid. Allsubstrate subsets where then heated in a quartz tube with an N₂ ambientfor 30 minutes to drive in the phosphorous dopant: a first subset washeated to 860° C., a second subset was heated to 900° C., and a thirdsubset was heated to 1000° C. All substrate subsets were then cleanedwith a 10 minute BOE and the sheet resistance under ink regions wasmeasured with a 4-point probe. In general, a 4-point probe determinesthe sample resistivity by supplying a high impedance current sourcethrough the outer two probes, and measuring voltage across the inner twoprobes.

At a drive-in temperature of 860° C., the sheet resistance of thesubstrate with Ferro paste is about 330 ohm/sq, while the sheetresistance of the substrate with HF doping paste is about 237 ohm/sq. Ata drive-in temperature of 900 ° C., the sheet resistance of thesubstrate with Ferro paste is about 372 ohm/sq, while the sheetresistance of the substrate with HF doping paste is about 161 ohm/sq.And at a drive-in temperature of 1000° C., the sheet resistance of thesubstrate with Ferro paste is about 224 ohm/sq, while the sheetresistance of the substrate with HF doping paste is about 75 ohm/sq. Ascan be seen, for any given temperature, a lower sheet resistance andthus a higher dopant concentration is driven into the substrate.

Experiment 4

Referring now to FIG. 8, a simplified diagram comparing the sheetresistance of various n-type high-fidelity doping paste configurationson a set of (2 ohm-cm/180 μm/saw damage etched) p-type siliconsubstrates, in accordance with the invention.

Sheet resistance (ohm/sq) 802 is shown along the vertical axis, whileexposure 804, doping concentration % 806, and drive-in temperature (°C.) 808, are shown along the horizontal axis.

As previously described, high-fidelity doping paste was prepared byaddition of 5, 10 or 18% of phosphoric acid to Si nanoparticles pastecontaining 1.5 wt % ethyl cellulose binder and 8 wt % siliconnanoparticles in a terpineol solvent, followed by thorough mixing with aplanetary mixer. The conventional (Ferro) doping paste was usedunmodified.

The set of p-type silicon substrates were each cleaned with ahydrofluoric acid/hydrochloric acid mixture prior to paste deposition.The substrate subsets were then baked at 200° C. for 3 minutes to removesolvent. All substrate subsets were then heated in a quartz tube with anN₂ ambient to dehydrate the phosphoric acid and to drive in thephosphorous dopant. Covered wafers were placed in the heated quartz tubeusing the back-to-back configuration described in FIG. 6. Exposed waferswere placed in the heated quartz tube with ink areas directly exposed tothe N₂ ambient.

At a drive-in temperature of 860° C., and a phosphorous dopingconcentration of 5%, the exposed sheet resistance of the substrate isabout 267 ohm/sq, while the covered sheet resistance of the substrate isabout 162 ohm/sq.

At a drive-in temperature of 860° C., and a phosphorous dopingconcentration of 10%, the exposed sheet resistance of the substrate isabout 237 ohm/sq, while the covered sheet resistance of the substrate isabout 127 ohm/sq.

At a drive-in temperature of 860° C., and a phosphorous dopingconcentration of 18%, the exposed sheet resistance of the substrate isabout 126 ohm/sq, while the covered sheet resistance of the substrate isabout 83 ohm/sq.

At a drive-in temperature of 900 ° C., and a phosphorous dopingconcentration of 5%, the exposed sheet resistance of the substrate isabout 198 ohm/sq, while the covered sheet resistance of the substrate isabout 112 ohm/sq.

At a drive-in temperature of 900° C., and a phosphorous dopingconcentration of 10%, the exposed sheet resistance of the substrate isabout 160 ohm/sq, while the covered sheet resistance of the substrate isabout 89 ohm/sq.

At a drive-in temperature of 900° C., and a phosphorous dopingconcentration of 18%, the exposed sheet resistance of the substrate isabout 89 ohm/sq, while the covered sheet resistance of the substrate isabout 57 ohm/sq.

At a drive-in temperature of 1000° C., and a phosphorous dopingconcentration of 5%, the exposed sheet resistance of the substrate isabout 114 ohm/sq, while the covered sheet resistance of the substrate isabout 42 ohm/sq.

At a drive-in temperature of 1000° C., and a phosphorous dopingconcentration of 10%, the exposed sheet resistance of the substrate isabout 75 ohm/sq, while the covered sheet resistance of the substrate isabout 52 ohm/sq.

At a drive-in temperature of 1000° C., and a phosphorous dopingconcentration of 18%, the exposed sheet resistance of the substrate isabout 36 ohm/sq, while the covered sheet resistance of the substrate isabout 41 ohm/sq.

As can be seen, for any given temperature, a lower sheet resistance andthus a higher dopant concentration is driven into the substrate.

Experiment 5

Referring now to FIGS. 9A-B, a set of simplified diagrams comparing thesheet resistance of various p-type (boron) high-fidelity doping pasteconfigurations on a set of (2 ohm-cm/180 μm/saw damage etched) n-typesilicon substrates, in accordance with the invention.

FIG. 9A displays the data in on a logarithmic scale, while FIG. 9Bdisplays the data on a linear scale.

Sheet resistance (ohm/sq) 902 is shown along the vertical axis, whileexposure 904, doping concentration % 906, and drive-in temperature (°C.) 908, are shown along the horizontal axis.

The high-fidelity doping paste was prepared by addition of 5, 10 or 19%of Triethyl borate to Si nanoparticles paste containing 1.5 wt % ethylcellulose binder and 8 wt % silicon nanoparticles in a terpineolsolvent, followed by thorough mixing with a planetary mixer.

Three high-fidelity doping paste boron concentrations where prepared(5%, 10%, and 19%). The set of n-type silicon substrates were eachcleaned with a hydrofluoric acid/hydrochloric acid mixture prior topaste deposition. The substrate subsets where then baked in N₂ ambientat 200° C. for 3 minutes to densify the films. All substrate subsetswhere then heated in a quartz tube with an N₂ ambient for 30 minutes todrive in the boron dopant.

At a drive-in temperature of 860° C., and a boron doping concentrationof 5%, the exposed sheet resistance of the substrate is about 1588ohm/sq, while the covered sheet resistance of the substrate is about 431ohm/sq.

At a drive-in temperature of 860° C., and a boron doping concentrationof 10%, the exposed sheet resistance of the substrate is about 889ohm/sq, while the covered sheet resistance of the substrate is about 268ohm/sq.

At a drive-in temperature of 860° C., and a boron doping concentrationof 19%, the exposed sheet resistance of the substrate is about 629ohm/sq, while the covered sheet resistance of the substrate is about 247ohm/sq.

At a drive-in temperature of 900° C., and a boron doping concentrationof 5%, the exposed sheet resistance of the substrate is about 1232ohm/sq, while the covered sheet resistance of the substrate is about 231ohm/sq.

At a drive-in temperature of 900° C., and a boron doping concentrationof 10%, the exposed sheet resistance of the substrate is about 603ohm/sq, while the covered sheet resistance of the substrate is about 171ohm/sq.

At a drive-in temperature of 900° C., and a boron doping concentrationof 19%, the exposed sheet resistance of the substrate is about 520ohm/sq, while the covered sheet resistance of the substrate is about 154ohm/sq.

At a drive-in temperature of 1000° C., and a boron doping concentrationof 5%, the exposed sheet resistance of the substrate is about 653ohm/sq, while the covered sheet resistance of the substrate is about 58ohm/sq.

At a drive-in temperature of 1000° C., and a boron doping concentrationof 10%, the exposed sheet resistance of the substrate is about 297ohm/sq, while the covered sheet resistance of the substrate is about 43ohm/sq.

At a drive-in temperature of 1000° C., and a boron doping concentrationof 19%, the exposed sheet resistance of the substrate is about 105ohm/sq, while the covered sheet resistance of the substrate is about 43ohm/sq.

As can be seen, for any given temperature, a lower sheet resistance andthus a higher dopant concentration is driven into the substrate.

The inventions illustratively described herein may suitably be practicedin the absence of any element or elements, limitation or limitations,not specifically disclosed herein. Thus, for example, the terms“comprising,” “including,” “containing,” etc. shall be read expansivelyand without limitation. Additionally, the terms and expressions employedherein have been used as terms of description and not of limitation, andthere is no intention in the use of such terms and expressions ofexcluding any equivalents of the features shown and described orportions thereof, but it is recognized that various modifications arepossible within the scope of the invention claimed.

Thus, it should be understood that although the present invention hasbeen specifically disclosed by preferred embodiments and optionalfeatures, modification, improvement and variation of the inventionsherein disclosed may be resorted to by those skilled in the art, andthat such modifications, improvements and variations are considered tobe within the scope of this invention. The materials, methods, andexamples provided here are representative of preferred embodiments, areexemplary, and are not intended as limitations on the scope of theinvention.

As will be understood by one skilled in the art, for any and allpurposes, particularly in terms of providing a written description, allranges disclosed herein also encompass any and all possible sub-rangesand combinations of sub-ranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” “greater than,” “less than,” and the likeinclude the number recited and refer to ranges which can be subsequentlybroken down into sub-ranges as discussed above. In addition, the terms“dopant or doped” and “counter-dopant or counter-doped” refer to a setof dopants of opposite types. That is, if the dopant is p-type, then thecounter-dopant is n-type. Furthermore, unless otherwise dopant-types maybe switched. In addition, the silicon substrate may be eithermono-crystalline or multi-crystalline. In addition, “undoped” refers toa material with a lack of dopant. As described herein, the ketonemolecules and the alcohol molecules may be cyclic, straight, orbranched.

Furthermore, this invention may be applied to other solar cellstructures as described in U.S. patent application Ser. No. 12/029,838,entitled Methods and Apparatus for Creating Junctions on a Substrate,filed Feb. 12, 2008, the entire disclosure of which is incorporated byreference.

All publications, patent applications, issued patents, and otherdocuments referred to in this specification are herein incorporated byreference as if each individual publication, patent application, issuedpatent or other document were specifically and individually indicated tobe incorporated by reference in its entirety. Definitions that arecontained in text incorporated by reference are excluded to the extentthat they contradict definitions in this disclosure.

For the purposes of this disclosure and unless otherwise specified, “a”or “an” means “one or more.” All patents, applications, references andpublications cited herein are incorporated by reference in theirentirety to the same extent as if they were individually incorporated byreference. In addition, the word set refers to a collection of one ormore items or objects.

Advantages of the invention include a high fidelity doping paste,optimized for screen printing in the high-volume manufacture of solarcells.

Having disclosed exemplary embodiments and the best mode, modificationsand variations may be made to the disclosed embodiments while remainingwithin the subject and spirit of the invention as defined by thefollowing claims.

1. A method of doping a semiconductor substrate, comprising: depositingon a surface of a semiconductor substrate a dopant paste comprising asolvent, a set of non-glass matrix particles dispersed into the solvent,and a dopant, wherein the set of non-glass matrix particles is a set ofceramic particles selected from the group consisting of Al₂O₃, MgO,CeO₂, TiO₂, ZnO, ZrO₂, ZrO₂₋₃, and Y₂O₃, wherein the dopant is selectedfrom the group consisting of phosphorous dopant, arsenic dopant,antimony dopant, boron dopant and gallium dopant; and heating the dopantpaste on the surface of the semiconductor substrate to diffuse thedopant into the semiconductor substrate.
 2. A method of claim 1, whereinthe dopant paste further comprises a binder.
 3. A method of claim 1,wherein the dopant constitutes 10-19 wt % of the paste.
 4. A method ofclaim 1, wherein the heating temperature is about 800° C. to 1050° C. 5.A method of claim 1, wherein the solvent is an organic solvent withboiling point greater than about 200° C.
 6. A method of claim 1, whereinthe solvent is selected from the group consisting of a solvent with alinear or cyclic structure, a solvent with saturated or unsaturatedhydrocarbon parts, a hydrocarbon-based solvent, an alcohol, a thiol, anether, an ester, an aldehyde, a ketone, and combinations thereof.
 7. Amethod of claim 5, wherein the binder is a polymer soluble in theorganic solvent.
 8. A method of claim 5, wherein the binder is one of apolyacrylate, a polyacetal, a polyvinyl, a cellulose, and copolymersthereof.
 9. A method of claim 1, wherein the dopant is an n-type dopantprecursor or a p-type dopant precursor.
 10. A method of claim 9, whereinthe n-type dopant precursor is one of an n-type liquid, an n-type solid,and an n-type polymer.
 11. A method of claim 10, where in n-type liquidis one of H₃PO₄ and organophosphate.
 12. A method of claim 10, whereinn-type solid is one of P₂O₅, Na₃PO₄, AlPO₄, AlP, and Na₃P.
 13. A methodof claim 10, wherein n-type polymer is one of a polyphosphonate and apolyphosphazene.
 14. A method of claim 9, wherein the p-type dopantprecursor is one of a p-type liquid, a p-type solid, a p-type binarycompound, and a p-type polymer.
 15. A method of claim 14, wherein thep-type liquid is B(OR)₃.
 16. A method of claim 14, wherein the p-typesolid is one of B(OH)₃, NaBO₂, Na₂B₄O₇, and B₂O₃.
 17. A method of claim14, wherein the p-type binary compound is one of boronitride, boroncarbide, boron silicide and elementary boron.
 18. A method of claim 14,wherein the p-type polymer is one of a polyborazole, and aorganoboron-silicon.
 19. A method of claim 1, wherein the averagediameter of the set of the non-glass matrix particles is less than 25microns.
 20. A method of claim 1, wherein the set of non-glass matrixparticles is a set of ceramic particles selected from the groupconsisting of TiO₂ and ZrO₂.
 21. A method of claim 1, wherein thesemiconductor substrate is a silicon substrate of a solar cell.
 22. Amethod of claim 1, wherein the paste is deposited in such a way thatupon said heating a doped pattern is formed on the substrate.
 23. Amethod of claim 22, wherein said doped pattern is a solar cell dopedpattern.